Photoelectrochemical water splitting at low applied potential using a NiOOH coated codoped (Sn, Zr) α-Fe2O3 photoanode

Abstract

We synthesized a NiOOH decorated codoped (Sn, Zr) α-Fe ₂O ₃ photoanode that results in enhanced photoelectrochemical performance and drastically lower onset potential.

Abstract

One of the major challenges in photoelectrochemical water splitting is to develop an efficient photoanode that can oxidize water at low applied potential. Herein, a codoped (Sn, Zr) α-Fe ₂O ₃ photoanode modified with a stable and earth abundant nickel oxyhydroxide (NiOOH) co-catalyst that can split water at low applied potential is reported. First, an unintentional gradient monodoped (Sn) α-Fe ₂O ₃ photoanode was synthesized at controlled annealing temperature that achieved a photocurrent density of 0.86 mA cm ⁻² at 1.23 V vs. RHE. Further doping with an optimized amount of Zr outperformed the monodoped (Sn) α-Fe ₂O ₃ photoanode providing significantly much higher photocurrent density (1.34 mA cm ⁻²). The remarkably improved electrical conductivity and more than three times higher charge carrier density (as evidenced from electrochemical impedance spectroscopy measurements and Mott–Schottky analysis) of the codoped (Sn, Zr) α-Fe ₂O ₃ photoanode highlight the importance of codoping. The synergetic effect of codoping (Sn, Zr) led to 1.6-fold enhancement in charge separation efficiency at 1.23 V compared to the monodoped (Sn) α-Fe ₂O ₃ photoanode. The NiOOH modified codoped (Sn, Zr) α-Fe ₂O ₃ photoanode exhibited drastically lower onset potential (0.58 V) and a photocurrent density of 1.64 mA cm ⁻² at 1.23 V. Interestingly a 160 mV cathodic shift in photocurrent onset potential was also observed. Concomitant with this, the NiOOH modified codoped (Sn, Zr) α-Fe ₂O ₃ photoanode exhibited 1.6 to 9.5-fold enhancement in charge injection efficiency ( η _inj) at the kinetic control region of 0.7 to 0.9 V compared to the unmodified codoped photoanode. Gas evolution measurements also showed that the NiOOH modified codoped α-Fe ₂O ₃ photoanode achieved an average Faradaic efficiency of 93%.